Scientists Address 'Wrinkles' in Transparent Film Development

A closer look at a promising nanotube coating that might one day improve solar
cells has turned up a few unexpected wrinkles, according to new research* conducted
at the National Institute of
Standards and Technology (NIST) and North Dakota State University (NDSU)-research
that also may help scientists iron out a solution.

This atomic-force microscopy image shows wrinkling in a single-wall carbon nanotube membrane; the inset shows an optical reflection micrograph of the membrane without any strain. The random arrangement of the nanotubes shown in the inset creates conductivity, but wrinkling can disrupt that. Each image is 40 micrometers in width. Credit: NIST

The scientists have found that coatings made of single-walled carbon nanotubes
(SWCNTs) are not quite as deformable as hoped, implying that they are not an
easy answer to problems that other materials present. Though films made of nanotubes
possess many desirable properties, the team’s findings reveal some issues
that might need to be addressed before the full potential of these coatings
is realized.

“The irony of these nanotube coatings is that they can change when they
bend,” says Erik Hobbie, now the director of the Materials and Nanotechnology
program at NDSU. “Under modest strains, these films can develop irreversible
changes in nanotube arrangement that reduce their conductivity. Our work is
the first to suggest this, and it opens up new approaches to engineering the
films in ways that minimize these effects.”

High on the wish list of the solar power industry is a cheap, flexible, transparent
coating that can conduct electricity. If this combination of properties can
somehow be realized in a single material, solar cells might become far less
expensive, and manufacturers might be able to put them in unexpected places—such
as articles of clothing. Transparent conductive coatings can be made of indium-tin
oxide, but their rigidity and high cost make them less practical for widespread
use.

Carbon nanotubes are one possible solution. Nanotubes, which resemble microscopic
rolls of chicken wire, are inexpensive, easy to produce, and can be formed en
masse into transparent conductive coatings whose weblike inner structure makes
them not only strong but deformable, like paper or fabric. However, the team’s
research found that some kinds of stretching cause microscopic ‘wrinkles’
in the coating that disrupt the random arrangement of the nanotubes, which is
what makes the coating conduct electricity.

“You want the nanotubes to stay randomly arranged,” Hobbie says.
“But when a nanotube coating wrinkles, it can lose the connected network
that gives it conductivity. Instead, the nanotubes bundle irreversibly into
ropelike formations.”

Hobbie says the study suggests a few ways to address the problem, however.
The films might be kept thin enough so the wrinkling might be avoided in the
first place, or designers could engineer a second interpenetrating polymer network
that would support the nanotube network, to keep it from changing too much in
response to stress. “These approaches might allow us to make coatings
of nanotubes that could withstand large strains while retaining the traits we
want,” Hobbie says.

Graphene oxide membranes have been receiving attention for their extremely powerful separation abilities and the ease at which it can be modified, allowing for membrane permittivity to be fine-tuned. These membranes show the potential to be used for water purification, ‘green’ gas purification and greenhouse gas capture.